Variable magnification viewfinder optical system

ABSTRACT

The present invention provides a variable magnification viewfinder optical system, comprising from the object side: objective lens group of a positive refractive power, including a first lens unit of a negative refractive power and a second lens unit of a positive refractive power, said first and second lens units being shiftable along the optical axis of the optical system for changing the magnification of the optical system; condenser lens group of a positive refractive power; and eyepiece lens group of a negative refractive power; wherein the objective lens group forms a real image of an object and the real image can be observed through the eyepiece lens group; and wherein the optical system fulfills the following conditions; 
     
         -0.09/mm&lt;1/f.sub.1 &lt;-0.03/mm 
    
     
         0.04/mm&lt;1/f.sub.2 &lt;0.11/mm 
    
     
         -0.1/mm&lt;fe/(f.sub.1 ·f.sub.2)&lt;-0.035/mm 
    
     wherein: f 1  represents focal length of the first lens unit of the objective lens group in millimeters; f 2  represents focal length of the second lens unit of the objective lens group in millimeters; and f e  represents focal length of the eyepiece lens group in millimeters.

This is a continuation of application Ser. No. 333,701, filed Apr. 4,1989 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a viewfinder optical system used in aphotographic camera or in a video camera.

2. Description of the Prior Art

A camera of a type having a viewfinder optical system independently of aphotographic optical system has been known. In this type of camera, theviewfinder optical system must alter its magnification if a zoom lenssystem is used as the photographic optical system. Therefore, aplurality of kinds of variable magnification viewfinder optical systemshave been proposed.

Japanese Laid-Open Patent Application No. 61-160712 proposes a variablemagnification viewfinder optical system of inverted Galilean type. Thevariable magnification viewfinder optical system, proposed by JapaneseLaid-Open Patent Application No. 61-160712, comprises from the objectside, an objective lens group of a negative refractive power including apositive front lens unit and a negative rear lens unit, and an eyepiecelens group of a positive refractive power, wherein the positive frontlens unit and the negative rear lens unit are independently shiftablealong the optical axis of the optical system for continuously alteringmagnification thereof while the eyepiece lens group is stationary on theoptical axis. Additionally, Japanese Laid-Open Patent Application No.61-160712 also proposes, in its embodiments, a combination of aninverted Galilean type finder lens system and an Albada finder system.Therefore, a virtual image of a photographic frame is formed by theAlbada finger system together with a virtual finder image formed by theinverted Galilean type finder lens system. Namely, in some embodimentsdisclosed in Japanese Laid-Open Patent application No. 61-160712, ahalf-mirror layer is provided on a rear surface of the negative rearlens unit of the objective lens group for reflecting light coming from aphotographic frame located behind the objective lens group.

However, the above-explained type finder optical system, which forms thevirtual finder image together with the virtual image of the photographicframe, has drawbacks in which a finder field covered by the virtualimage of the photographic frame may be changed in accordance with thechange in position of the observer's eye, and in that the virtual imageof the photographic frame may be blurred in altering the findermagnification. It should be also necessary to design the optical systemso as to prevent the occurrence of any ghost and flare due to thehalf-mirror. Additionally, the diameter of the objective lens group mayhave to become large for obtaining more change in the findermagnification.

On the other hand, the other type viewfinder optical system has alsobeen known in which a real image of an object is formed by an objectivelens group of a positive refractive power and a photographic frame islocated at the position on which the real image of the object is formedby the objective lens group, and in which the real image and thephotographic frame are observed through an eyepiece lens group locatedat the rear side of the photographic frame. In this type of viewfinderoptical system, the photographic frame can be observed clearly and acorrect finder field can be indicated by the photographic frame even ifthe position of the observer's eye is changed.

Japanese Laid-Open Patent Application No. 61-156018 proposes a variablemagnification viewfinder optical system comprising, from the objectside, an objective lens group of a positive refractive power including afirst negative lens unit, a second positive lens unit and a thirdpositive lens unit, and an eyepiece lens group of a positive refractivepower, wherein a real image of an object is formed nearby the third lensunit by the first and second lens units, and wherein the first andsecond lens units are shiftable for continuously altering the findermagnification.

However, the variable magnification viewfinder optical system proposedby Japanese Laid-Open Patent Application No. 61-156018 has aninsufficient change of the finder magnification. Additionally, thevariable magnification viewfinder optical system proposed by JapaneseLaid-Open Patent Application No. 61-156018 is severe on errors caused inmanufacturing each of the lens elements contained in each lens groupsand in assembling them to complete an optical system since therespective lens groups are designed to have relatively strong refractivepowers, although the total length of the whole optical system iscompact.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a variablemagnification viewfinder optical system in which a real image of anobject is once formed therein, and having a sufficient change in findermagnification.

Another object of the present invention is to provide a variablemagnification viewfinder optical system having a high ratio of changingthe finder magnification.

A further other object of the present invention is to provide theabove-noted variable magnification viewfinder optical system in which animage of the photographic frame can be clearly observed, and in which acorrect finder field can be indicated by the photographic frame even isthe position of the observer's eye is changed.

Another object of the present invention is to provide a variablemagnification viewfinder optical system having a high ratio of changingthe finder magnification reaching about 2.0 and a minimum findermagnification of 0.4.

The achieve the above objects, according to the present invention, avariable magnification viewfinder optical system, comprises from theobject side: an objective lens group of a positive refractive power,including a first lens unit of a negative refractive power and a secondlens unit of a positive refractive power, said first and second lensunits being shiftable along the optical axis of the optical system forchanging the magnification of the optical system; a condenser lens groupof a positive refractive power; and an eyepiece lens group of a negativerefractive power; wherein the objective lens group forms an actual imageof an object and the actual image can be observed through the eyepiecelens group; and wherein the optical system fulfills the followingconditions;

    0.09/mm<1/f.sub.1 <-0.03/mm

    0.04/mm<1/f.sub.2 <0.11/mm

    -0.1/mm<f.sub.e /(f.sub.1.f.sub.2)<-0.035mm

wherein:

f₁ represents a focal length of the first lens unit of the objectivelens group in millimiters;

f₂ represents focal a length of the second lens unit of the objectivelens group in millimeters; and

f_(e) represents a focal length of the eyepiece lens group inmillimeters.

DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a cross sectional view of a first embodiment accordingto the present invention in minimum magnification condition;

FIG. 2 represents a cross sectional view of the first embodimentaccording to the present invention in middle magnification condition;

FIG. 3 represents a cross sectional view of the first embodimentaccording to the present invention in maximum magnification condition;

FIG. 4 represents a cross sectional view of a second embodimentaccording to the present invention in minimum magnification condition;

FIG. 5 represents a cross sectional view of the second embodimentaccording to the present invention in middle magnification condition;

FIG. 6 represents a cross sectional view of the second embodimentaccording to the present invention in maximum magnification condition;

FIG. 7 represents a cross sectional view of a third embodiment accordingto the present invention in minimum magnification condition;

FIG. 8 represents a cross sectional view of the third embodimentaccording to the present invention in middle magnification condition;

FIG. 9 represents a cross sectional view of the third embodimentaccording to the present invention in maximum magnification condition;

FIG. 10 represents a cross sectional view of a fourth embodimentaccording to the present invention in minimum magnification condition;

FIG. 11 represents a cross sectional view of the fourth embodimentaccording to the present invention in middle magnification condition;

FIG. 12 represents a cross sectional view of the fourth embodimentaccording to the present invention in maximum magnification condition;

FIG. 13 represents a cross sectional view of a fifth embodimentaccording to the present invention in minimum magnification condition;

FIG. 14 represents a cross sectional view of the fifth embodimentaccording to the present invention in middle magnification condition;

FIG. 15 represents a cross sectional view of the fifth embodimentaccording to the present invention in maximum magnification condition;

FIG. 16 represents a cross sectional view of a sixth embodimentaccording to the present invention in minimum magnification condition;

FIG. 17 represents a cross sectional view of the sixth embodimentaccording to the present invention in middle magnification condition;

FIG. 18 represents a cross sectional view of the sixth embodimentaccording to the present invention in maximum magnification condition;

FIG. 19 represents a cross sectional view of a seventh embodimentaccording to the present invention in minimum magnification condition;

FIG. 20 represents a cross sectional view of the seventh embodimentaccording to the present invention in middle magnification condition;

FIG. 21 represents a cross sectional view of the seventh embodimentaccording to the present invention in maximum magnification condition;

FIG. 22 represents a cross sectional view of a eighth embodimentaccording to the present invention in minimum magnification condition;

FIG. 23 represents a cross sectional view of the eighth embodimentaccording to the present invention in middle magnification condition;

FIG. 24 represents a cross sectional view of the eighth embodimentaccording to the present invention in maximum magnification condition;

FIG. 25 represents a cross sectional view of a ninth embodimentaccording to the present invention in minimum magnification condition;

FIG. 26 represents a cross sectional view of the ninth embodimentaccording to the present invention in middle magnification condition;

FIG. 27 represents a cross sectional view of the ninth embodimentaccording to the present invention in maximum magnification condition;

FIG. 28 represents a cross sectional view of a tenth embodimentaccording to the present invention in minimum magnification condition;

FIG. 29 represents a cross sectional view of the tenth embodimentaccording to the present invention in middle magnification condition;

FIG. 30 represents a cross sectional view of the tenth embodimentaccording to the present invention in maximum magnification condition;

FIGS. 31a to 31c represent the aberration curves of the first embodimentin minimum magnification condition;

FIGS. 32a to 32c represent the aberration curves of the first embodimentin middle magnification condition;

FIGS. 33a to 33c represent the aberration curves of the first embodimentin maximum magnification condition;

FIGS. 34a to 34c represent the aberration curves of the secondembodiment in minimum magnification condition;

FIGS. 35a to 35c represent the aberration curves of the secondembodiment in middle magnification condition;

FIGS. 36a to 36c represent the aberration curves of the secondembodiment in maximum magnification condition;

FIGS. 37a to 37c represent the aberration curves of the third embodimentin minimum magnification condition;

FIGS. 38a to 38c represent the aberration curves of the third embodimentin middle magnification condition;

FIGS. 39a to 39c represent the aberration curves of the third embodimentin maximum magnification condition;

FIGS. 40a to 40c represent the aberration curves of the fourthembodiment in minimum magnification condition;

FIGS. 41a to 41c represent the aberration curves of the fourthembodiment in middle magnification condition;

FIGS. 42a to 42c represent the aberration curves of the fourthembodiment in maximum magnification condition;

FIGS. 43a to 43c represent the aberration curves of the fifth embodimentin minimum magnification condition;

FIGS. 44a to 44c represent the aberration curves of the fifth embodimentin middle magnification condition;

FIGS. 45a to 45c represent the aberration curves of the fifth embodimentin maximum magnification condition;

FIGS. 46a to 46c represent the aberration curves of the sixth embodimentin minimum magnification condition;

FIGS. 47a to 47c represent the aberration curves of the sixth embodimentin middle magnification condition;

FIGS. 48a to 48c represent the aberration curves of the sixth embodimentin maximum magnification condition;

FIGS. 49a to 49c represent the aberration curves of the seventhembodiment in minimum magnification condition;

FIGS. 50a to 50c represent the aberration curves of the seventhembodiment in middle magnification condition;

FIGS. 51a to 51c represent the aberration curves of the seventhembodiment in maximum magnification condition;

FIGS. 52a to 52c represent the aberration curves of the eighthembodiment in minimum magnification condition;

FIGS. 53a to 53c represent the aberration curves of the eighthembodiment in middle magnification condition;

FIGS. 54a to 54c represent the aberration curves of the eighthembodiment in maximum magnification condition;

FIGS. 55a to 55c represent the aberration curves of the ninth embodimentin minimum magnification condition;

FIGS. 56a to 56c represent the aberration curves of the ninth embodimentin middle magnification condition;

FIGS. 57a to 57c represent the aberration curves of the ninth embodimentin maximum magnification condition;

FIGS. 58a to 58c represent the aberration curves of the tenth embodimentin minimum magnification condition;

FIGS. 59a to 59c represent the aberration curves of the tenth embodimentin middle magnification condition;

FIGS. 60a to 60c represent the aberration curves of the tenth embodimentin maximum magnification condition;

FIG. 61 represents a perspective view of the second embodiment inminimum magnification condition;

FIG. 62 represents a perspective view of the second embodiment in middlemagnification condition;

FIG. 63 represents a perspective view of a polo prism system usable inthe present invention;

FIG. 64 represents a perspective view of polo mirror system usable inthe present invention; and

FIG. 65 represents a schematic view of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled inthe optical and camera field to make and use the invention and setsforth the best modes contemplated by the inventors for carrying outtheir invention. Various modification, however, will remain readilyapparent to those skilled in the art, since the generic principles ofthe present invention have been defined herein specifically to provide arelatively economical and easily manufactured compact viewfinder opticalsystem. The derivation of the formulas and the relation of the powersset forth herein can be accomplished with the assistance of a computer.The present invention represents the parameters of a compromise balanceof acceptable aberrations which can be relatively easily manufactured toprovide a low cost viewfinder optical system.

In the drawings, schematic cross sectional views disclose the positionof the lens groups and the lens elements in the minimum, middle andmaximum magnification conditions.

As shown in FIGS. 1 to 30, the present invention provides a variablemagnification viewfinder optical system, comprising from the objectside: objective lens group (Lo) of a positive refractive power,including a first lens unit (Lo₁) of a negative refractive power and asecond lens unit (Lo₂) of a positive refractive power, said first andsecond lens units (Lo₁) and (Lo₂) being shiftable along the optical axisof the optical system for changing the magnification of the opticalsystem; condenser lens group (Lc) of a positive refractive power; andeyepiece lens group (Le) of a negative refractive power; wherein theobjective lens group (Lo) forms a real image of an object and the realimage can be observed through the eyepiece lens group (Le); and whereinthe optical system fulfills the following conditions;

    -0.09/mm<1/f.sub.1 <-0.03/mm                               (1)

    0.04/mm<1/f.sub.2 <0.11/mm                                 (2)

    -0.1/mm<f.sub.e /(f.sub.1 ·f.sub.2)<-0.035/mm     (3)

wherein:

f₁ represents focal length of the first lens unit (Lo₁) of the objectivelens group in millimeters;

f₂ represents focal length of the second lens unit (Lo₂) of theobjective lens group in millimeters; and

f_(e) represents focal length of the eyepiece lens group (Le) inmillimeters.

Initially, the principle of the present invention will be explained withreference to FIG. 65. In FIG. 65, (Im) represents a position on which areal image of an object is formed by the objective lens group (Lo). Inthe variable magnification viewfinder optical system in which a realimage of an object is once formed therein, an erecting system, forinverting the real image into an erect image, can not be located in aspace in which any movable lens group is shiftable for altering thefinder magnification. Contrary, the eyepiece lens group (Le) should bedesigned so that any aberrations generated by the eyepiece lens group(Le) are corrected well within the eyepiece lens group (Le). Therefore,it is impossible to shorten the focal length of the eyepiece lens group(Le) over a predetermined range.

If the erecting system is located at a space formed between the realimage position (Im) and the eyepiece lens group (Le), the findermagnification of the whole optical system decreases because the focallength of the eyepiece lens group (Le) should be extended. In such asituation, if the focal length of the objective lens group (Lo) isextended, the total length of the whole optical system should beextended since the back focal length of the objective lens group (Lo) isalso extended. The extension of the total length of the whole opticalsystem can be controlled by shortening both of the focal length of theeyepiece lens group (Le) and the focal length of the objective lensgroup (Lo). However, it becomes difficult to well correct all of theaberrations by shortening both focal lengths over their respectivelimits. The respective limits of the focal lengths of the eyepiece lensgroup (Le) and the objective lens group (Lo) for maintaining wellcorrected aberrations must be extended in accordance with an increase ofthe desirable minimum finder magnification and an increase of thedesirable ratio of change of finder magnification.

On the other hand, the amount of shift of the objective lens group (Lo),required for obtaining a sufficient ratio of change of findermagnification, should be increased, if the sufficient ratio of change offinder magnification is achieved only by the shift of the objective lensgroup (Lo). Here, it is easy to correct the aberrations by extending thefocal length of the objective lens group (Lo). However, this causes anextension of the total length of the whole optical system by theextension of the back focal length of the objective lens group (Lo) inaddition to the increase of the amount of shift thereof.

In the present invention, the objective lens group (Lo) is divided into,from the object side, the first negative lens unit (Lo₁) and the secondpositive lens unit (Lo₂) for extending the back focal length of theobjective lens group (Lo), and a space for locating the image erectingsystem is provided in front of and behind the condenser lens group (Lc).In the present invention, if β₂ represents the lateral magnification ofthe second lens unit (Lo₂), the range of changing the findermagnification is set at both sides of β₂ =-1, so that the total lengthof the objective lens group (Lo) should not be changed excessively. Insuch condition, the relation among the focal lengths of the first andsecond lens units (Lo₁) and (Lo₂), the focal length of the eyepiece lensgroup (Le) are determined for achieving a sufficient ratio of change offinder magnification reaching 2.0, and a minimum finder magnification of0.4. According to the present invention, if four mirrors (PM₁), (PM₂),(PM₃) and (PM₄) as shown in FIG. 64 are used as the erecting system, thecondenser lens group (Lc) can be located in front of the mirror (PM₁),between the mirrors (PM₁) and (PM₂), between the mirrors (PM₂) and (PM₃)as shown in FIGS. 61 and 62, between the mirrors (PM₃) and (PM₄), andbehind the mirror (PM₄).

Condition (1) limits the refractive power of the first lens unit (Lo₁).If the upper limit of the condition (1) is violated, the focal length ofthe objective lens group (Lo) becomes undesirably long, and this causesan increase of the amount of shift of the objective lens group (Lo) inaddition to the undesirable extension of the total length of the wholeoptical system. Contrary, if the lower limit of the condition (1) isviolated, it becomes difficult to well correct the aberrations since thefocal length of the objective lens group (Lo) becomes excessively short.It causes a difficulty for achieving a sufficient ratio of change offinder magnification reaching 2.0 with maintaining the well correctedaberrations. Especially, it becomes difficult to well correct thedistortion.

Condition (2) limits the refractive power of the second lens unit (Lo₁).If the upper limit of the condition (2) is violated, it becomesdifficult to well correct the aberrations since the focal length of theobjective lens group (Lo) becomes excessively short. It causesdifficulty in achieving a sufficient ratio of change of findermagnification reaching 2.0 with maintaining well corrected aberrations.Contrary, if the lower limit of the condition (1) is violated, the focallength of the objective lens group (Lo) becomes undesirably long, andthis causes an increase of the amount of shift of the objective lensgroup (Lo) in addition to the undesirable extension of the total lengthof the whole optical system.

Condition (3) is provided for limiting a relation between the focallengths of the first and second lens units (Lo₁) and (Lo₂) and that ofthe eyepiece lens group (Le) in order to achieve a sufficient ratio ofchange of finder magnification reaching 2.0 and the minimum findermagnification of 0.4 while maintaining well corrected aberrations. Ifthe upper limit of the condition (3) is violated, it becomes difficultto well correct the aberrations, although it becomes easy to increasethe minimum finder magnification. Especially, if the upper limit of thecondition (3) is violated by shortening the focal length of the eyepiecelens group (Le), it becomes difficult to well correct the aberrationswhile keeping a good balance therebetween. If the lower limit of thecondition (3) is violated, it becomes difficult to well correct theaberrations with achieving the sufficient ratio of change of findermagnification reaching 2.0.

It is desirable to further fulfill the following conditions in order toachieve the sufficient ratio of change of finder magnification reaching2.0 with maintaining the minimum finder magnification of 0.4:

    |r.sub.1B /r.sub.1F |<6                  (4)

    |r.sub.2B /r.sub.2F |<2                  (5)

    0.9<f.sub.oT /f.sub.e <3                                   (6)

wherein:

r_(1B) represents a radius of curvature of a lens surface located at theobject side in the first lens unit of the objective lens group;

r_(1F) represents a radius of curvature of a lens surface located at theeye side in the first lens unit of the objective lens group;

r_(2B) represents a radius of curvature of a lens surface located at theobject side in the second lens unit of the objective lens group;

r_(2F) represents a radius of curvature of a lens surface located at theeye side in the second lens unit of the objective lens group; and

f_(oT) represents focal length of the objective lens group at thelargest magnification condition.

If either of the upper limit of condition (4) and the lower limitthereof is violated, it is not suitable for the practical use since thedifference between the distortion generated in the minimum findermagnification condition and that in the maximum finder magnificationcondition in addition to the undesirable decrease of the findermagnification.

If either of the upper limit of condition (5) and the lower limitthereof is violated, the difference between the astigmatism generated inthe minimum finder magnification condition and that in the maximumfinder magnification condition becomes excessively large, this causesthe difficulty of correcting the astigmatism and the distortion whilekeeping a good balance therebetween in the entire finder magnificationrange and with achieving a sufficient high finder magnification ratio.

Condition (6) is determined for the purpose of maintaining the minimumfinder magnification of 0.4 with keeping the sufficient high findermagnification ratio. If the upper limit of condition (6) is violated,since the finder magnification become too large beyond the sufficientrange, it becomes difficult to achieve the good optical performance ofthe viewfinder optical system with keeping the sufficient high findermagnification ratio. Contrary, if the lower limit of condition (6) isviolated, it is not favorable for the practical use because the minimumfinder magnification becomes excessively small.

With respect to the conditions (4) and (6), more desirable ranges are asfollows:

    |r.sub.1B /r.sub.1F |<3                  (4')

    1.1<f.sub.oT /f.sub.e <3                                   (6')

It is further desirable to constitute the condenser lens group (Lc) by asingle positive lens element and a single negative lens element and tofulfill the following conditions:

    ν.sub.eP >45                                            (7)

    ν.sub.eN <40                                            (8)

    0.5<r.sub.eP /r.sub.eN <0.95                               (9)

    -0.8<r.sub.eB /f.sub.e <-0.35                              (10)

    -0.9<f.sub.eN /f.sub.e <-0.4                               (11)

wherein:

ν_(eP) represents an Abbe number of the positive lens element includedin the eyepiece lens group (Le);

ν_(eN) represents an Abbe number of the negative lens element includedin the eyepiece lens group (Le);

r_(eP) represents a radius of curvature of a lens surface of thepositive lens element faced to the negative lens element;

r_(eN) represents a radius of curvature of a lens surface of thenegative lens element faced to the positive lens element;

r_(eB) represents radius of curvature of a lens surface located at theeye side in the eyepiece lens group (Le); and

f_(eN) represents focal length of the negative lens element of theeyepiece lens group (Le).

Conditions (7) and (8) are determined for controlling the chromaticaberration generated in the eyepiece lens group (Le) and that generatedin the whole optical system in order to preventing any colored blur of aphotographic frame indicated in the finder image and that of the finderimage itself. If either of the conditions (7) and (8) is violated, itbecomes difficult to well correct the chromatic aberration generated inthe eyepiece lens group (Le) and that generated in the whole opticalsystem.

Condition (9) limits a ratio of a radius of curvature of a lens surfacefaced to the negative lens element in the positive lens element to aradius of curvature of a lens surface faced to the positive lens elementin the negative lens element, in order to cancel any aberrationsgenerated by one of both lens surfaces by corresponding aberrationsgenerated by another lens surface with each other.

Condition (10) limits a radius of curvature of a lens surface located atthe eye side in the eyepiece lens group (Le) in relation to a focallength of the negative lens element of the eyepiece lens group (Le). Ifthe upper limit of the condition (10) is violated, the astigmatism isdeteriorated in addition to the decrease of the finder magnification. Ifthe lower limit of the condition (10) is violated, both of thedistortion and the astigmatism are deteriorated. In both cases, itbecomes difficult to well correct aberrations since either of theaberrations is deteriorated over a correctable range.

Condition (11) defines a ratio between a focal length of the negativelens in the eyepiece lens group (Le) and a compound focal length of theeyepiece lens group (Le). Namely, the condition (11) is effective forwell correcting chromatic aberrations generated both in the eyepiecelens group (Le) and in the whole optical system.

It is further desirable to apply at least one aspheric surface to theobjective lens group (Lo) for further correcting any aberrations. Thefield curvature generated by the objective lens group (Lo) is furthercorrected by introducing at least one aspheric surface to the objectivelens group (Lo). Contrary, it is also desirable to apply, to thecondenser lens group (Lc), at least one aspheric surface whose radius ofcurvature increases in accordance with the increase of the distancemeasured from the optical axis to the lens surface to which the asphericsurface is applied, for well correcting the distortion and forcontrolling the change of the distortion all over the variablemagnification range. Furthermore, it is also desirable to apply, to theeyepiece lens group (Le), at least one aspheric surface whose radius ofcurvature increases in accordance with the increase of the distancemeasured from the optical axis to the lens surface to which the asphericsurface is applied, for well correcting the distortion, the astigmatism,and the spherical aberration.

The following Tables 1 to 10 disclose, respectively, the first throughtenth embodiments of the present invention. In the Tables, r is theradius of curvature with respective sub number indicating the surfacesfrom the object to eye side along the optical axis, d represents theaxial distance and includes both air spaces and the actual thickness ofthe lens elements along the optical axis, N equals the refractive indexand again, the sub numbers refer to the particular optical element fromthe object to the eye side, and finally, ν equals the Abbe number andthe sub numbers refer to the specific lens elements from the object tothe eye side. The asterisk (*) represents the aspheric surface, and itscoefficients are shown in respective Tables.

Here, the shape of the aspheric surface is represented by the followingequation, if a point designated on the aspheric surface is expressed by(X,Y,Z); ##EQU1## wherein:

X represents a distance measured along the optical surface from a toppoint on which the optical axis intersects the aspheric surface;

Φ represents a distance measured perpendicularly to the optical axisfrom the top point, that is Φ=(Y² +Z²)^(1/2) ;

C₀ represents a radius of curvature of the aspheric surface on the toppoint;

ε represents an aspheric coefficient of the aspheric surface; and

Ai represents a curvature of the aspheric surface of i-the order.

In all of the embodiments shown in Tables 1 to 10, Ai are defined byzero, and the shape of the aspheric surface is represented by theaspheric coefficient ε. However, the aspheric coefficient ε can beexpanded to a plurality of Ai.

The space formed between the objective lens group (Lo) and the condenserlens group (Lc) and the space formed between the condenser lens group(Lc) and the eyepiece lens group (Le) are filled with air in all of theembodiments shown in Tables 1 to 10. However, if the viewfinder opticalsystem of either of the embodiments is actually mounted on a camerabody, it is desirable to use a reflection mirror or a parro prism asshown in FIG. 63 for inverting a real image formed by the objective lensgroup (Lo). In the embodiments, the objective lens group (Lo) forms areal image of an object on a position apart from the condenser lensgroup (Lc) by 1 mm-1.5 mm. It is effective for preventing from thedifficulty of finder observation due to any dust attached to the lenssurface of the condenser lens group (Lc). In the embodiments, thecondenser lens group (Lc) consists of a single lens element. However, itis also favorable to constitute the condenser lens group (Lc) by aplurality of lens elements if used with the polo prism. The objectivelens group (Lo) forms a real image of an object behind the condenserlens group (Lc) in all of the embodiments except for the embodiment 3.However, the position on which the real image is formed should bedefined while providing space into which the viewfinder optical systemshould be included and the arrangement thereof. It is also desirable toseal up any spaces formed between successive fixed lens groups in orderto avoiding any enter of the dust into the spaces. For example, if thereal image is formed behind the condenser lens group (Lc), it is onlynecessary for sealing up the space formed between the condenser lensgroup (Lc) and the eyepiece lens group (Le) to avoid any enter of thedust into the space after the viewfinder optical system is once mountedon the camera body.

In the embodiments, a spaced formed between two successive lens elementsin the eyepiece lens group (Le) can be changed for controlling thediopter within a sufficient adjustable range, without changing thefinder magnification. In this case, it is possible to shift one of thepositive and negative lens element for changing the space while makinganother of the positive and negative lens element stationary. Opticalperformance achieved by shifting the negative lens element is almostidentical to that achieved by shifting the positive lens element. Forexample, in a viewfinder optical system of the following embodiment 1, adiopter in the center of finder field and finder magnification when thepositive lens element (G₆) is shifted for controlling the diopter arerepresented in the following Table 11. As shown in the Table 11, thediopter can be controlled within a broad range of -4.3 diopter to +2.6diopter.

Diopter is defined by the reciprocal of the focal length of the lenssystem. In the present invention, the real image of an object is formedadjacent the condenser lens group by the objective lens group having apositive refractive power and an enlarged virtual image of this realimage is formed by the eyepiece lens group having a negative refractivepower. In the following Tables, the term "diopter in center" refers to adiopter value being calculated by tracing the paraxial ray through theoptical system.

As can be readily appreciated, it is possible to deviate from thefollowing embodiments of the present invention and, as will be readilyunderstood by those skilled in the optical art, the invention is capableof many modifications and improvements within the scope and spiritthereof. Accordingly, it will be understood that the invention is not tobe limited by these specific embodiments, but only by the scope andspirit of the appended claims.

                                      TABLE 1                                     __________________________________________________________________________    Embodiment 1                                                                  __________________________________________________________________________         Radius of                                                                            Axial  Refractive                                                                          Abbe  Aspheric                                            Curvature                                                                            Distance                                                                             Index (Nd)                                                                          Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 -42.13            -4.000107                                      Lo.sub.1 G1 d.sub.1 1.148                                                                        N.sub.1 1.713                                                                       ν.sub.1 53.93                                          r.sub.2 46.17                                                                        d.sub.2 variable                                                       r.sub.3 24.515            -3.608165                                      G.sub.2     d.sub.3 5.0                                                                          N.sub.2 1.4914                                                                      ν.sub.2 57.32                                          r.sub.4 -13.289                                                          Lo.sub.2    d.sub.4 0.5                                                            r.sub.5 -9.178                                                           G.sub.3     d.sub.5 1.5                                                                          N.sub.3 1.584                                                                       ν.sub.3 31.00                                          r.sub.6 -10.782                                                                      d.sub.6 variable                                                       r.sub.7 16.809            -1.316301                                      Lc G.sub.4  d.sub.7 3.4                                                                          N.sub.4 1.4914                                                                      ν.sub.4 57.32                                          r.sub.8 -57.466                                                                      d.sub.8 25.5                                                           r.sub.9 -94.616                                                          G.sub.5     d.sub.9 1.3                                                                          N.sub.5 1.584                                                                       ν.sub. 5 31.00                                         r.sub.10 9.209                                                           Le          d.sub.10 0.5                                                           r.sub.11 8.000            -0.0075                                        G.sub.6     d.sub.11 3.7                                                                         N.sub.6 1.4914                                                                      ν.sub.6 57.82                                          r.sub.12 -13.181                                                                     d.sub.12 13.0                                                          r.sub.13 pupil                                                           __________________________________________________________________________                           diopter                                                                            finder                                                      d.sub.2                                                                           d.sub.6  in center                                                                          magnification                                     __________________________________________________________________________    Standard  26.596                                                                            27.562   -0.909                                                                             0.508                                             Middle    7.024                                                                             39.572   -0.909                                                                             1.016                                             Tele      0.5 51.582   -0.909                                                                             1.524                                             frame                  -1.024                                                 __________________________________________________________________________                  1/f.sub.1                                                                          = -0.0325                                                                1/f.sub.2                                                                          = 0.0461                                                                 f.sub.e /(f.sub.1 · f.sub.2)                                              = -0.0451                                                                r.sub.1B /r.sub.1F                                                                 = -1.0959                                                                r.sub.2B /r.sub.2F                                                                 = -0.4398                                                                f.sub.oT /f.sub.e                                                                  = 1.7006                                                                 ν eP                                                                            = 57.82                                                                  ν eN                                                                            = 31.00                                                                  r.sub.eP /r.sub.eN                                                                 = 0.8687                                                                 r.sub.eB /f.sub.e                                                                  = -0.4390                                                                f.sub.eN /f.sub.e                                                                  = -0.4764                                                  __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Embodiment 2                                                                  __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 -24.00            -25.00                                         Lo.sub.1 G1 d.sub.1 1.5                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 34.7955                                                                      d.sub.2 variable                                                       r.sub.3 13.5              -4.9                                           Lo.sub.2 G.sub.2                                                                          d.sub.3 2.5                                                                         N.sub.2 1.4914                                                                       ν.sub.2 57.82                                          r.sub.4 -17.912                                                                      d.sub.4 variable                                                       r.sub.5 13.00             -0.8                                           Lc G.sub.3  d.sub.5 3.5                                                                         N.sub.3 1.4914                                                                       ν.sub.3 57.82                                          r.sub.6 -200.00                                                                      d.sub.6 24.5                                                           r.sub.7 -55.00                                                           G.sub.4     d.sub.7 1.5                                                                         N.sub.4 1.584                                                                        ν.sub.4 31.00                                          r.sub.8 12.00                                                            Le          d.sub.8 0.5                                                            r.sub.9 9.5               -0.7                                           G.sub.5     d.sub.9  3.0                                                                        N.sub.5 1.4914                                                                       ν.sub.5 57.82                                          r.sub.10 -12.78                                                                      d.sub.10 13.0                                                          r.sub.11 pupil                                                           __________________________________________________________________________                          diopter                                                                            finder                                                       d.sub.2                                                                          d.sub.4  in center                                                                          magnification                                      __________________________________________________________________________    Standard  19.2                                                                             20.352   -1.000                                                                             0.416                                              Middle    4.95                                                                             29.431   -1.000                                                                             0.831                                              Tele      0.2                                                                              38.509   -1.000                                                                             1.247                                              frame                 -1.000                                                  __________________________________________________________________________                  1/f.sub.1                                                                          = -0.0415                                                                1/f.sub.2                                                                          = -0.0622                                                                f.sub.e /(f.sub.1 · f.sub.2)                                              = -0.0749                                                                r.sub.1B /r.sub.1F                                                                 = -1.4498                                                                r.sub.2B /r.sub.2F                                                                 = -1.3268                                                                f.sub.oT /f.sub.e                                                                  = 1.4061                                                                 ν eP                                                                            = 57.82                                                                  ν eN                                                                            = 31.00                                                                  r.sub.eP /r.sub.eN                                                                 = 0.7917                                                                 r.sub.eB /f.sub.e                                                                  = -0.4405                                                                f.sub.eN /f.sub.e                                                                  = -0.5766                                                  __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Embodiment 3                                                                  __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 -21.00            -8.00                                          Lo.sub.1 G1 d.sub.1 1.0                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 37.4935                                                                      d.sub.2 variable                                                       r.sub.3 14.0              -4.6                                           Lo.sub.2 G.sub.2                                                                          d.sub.3 2.5                                                                         N.sub.2 1.4914                                                                       ν.sub.2 57.82                                          r.sub.4 -17.63                                                                       d.sub.4 variable                                                       r.sub.5 14.00             -1.2                                           Lc G.sub.3  d.sub.5 3.5                                                                         N.sub.3 1.4914                                                                       ν.sub.3 57.82                                          r.sub.6 -100.00                                                                      d.sub.6 20.07                                                          r.sub.7 -100.00                                                          G.sub.4     d.sub.7 1.3                                                                         N.sub.4 1.584                                                                        ν.sub.4 31.00                                          r.sub.8 9.3                                                              Le          d.sub.8 0.5                                                            r.sub.9 8.0               0.01                                           G.sub.5     d.sub.9 3.6                                                                         N.sub. 5 1.4914                                                                      ν.sub.5 57.82                                          r.sub.10 -13.17                                                                      d.sub.10 13.0                                                          r.sub.11 pupil                                                           __________________________________________________________________________                          diopter                                                                            finder                                                       d.sub.2                                                                          d.sub.4  in center                                                                          magnification                                      __________________________________________________________________________    Standard  19.0                                                                              26.277  -1.001                                                                             0.511                                              Middle    5.7                                                                              36.09    -0.999                                                                             1.016                                              Tele      1.2                                                                              46.0     -1.000                                                                             1.525                                              frame                 -1.000                                                  __________________________________________________________________________                  1/f.sub.1                                                                          = -0.0437                                                                1/f.sub.2                                                                          = -0.0613                                                                f.sub.e /(f.sub.1 · f.sub.2)                                              = -0.0784                                                                r.sub.1B /r.sub.1F                                                                 = -1.7854                                                                r.sub.2B /r.sub.2F                                                                 = -1.2593                                                                f.sub.oT /f.sub.e                                                                  = 1.4221                                                                 ν eP                                                                            = 57.82                                                                  ν eN                                                                            = 31.00                                                                  r.sub.eP /r.sub.eN                                                                 = 0.8602                                                                 r.sub.eB /f.sub.e                                                                  = -0.4496                                                                f.sub.eN /f.sub.e                                                                  = -0.4952                                                  __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Embodiment 4                                                                  __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 29.202                                                           G.sub.1     d.sub.1 1.2                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 158.158           0.641787                                       Lo.sub.1    d.sub.2 2.0                                                            r.sub.3 -35.114                                                          G.sub.2     d.sub.3 1.2                                                                         N.sub.2 1.584                                                                        ν.sub.2 31.00                                          r.sub.4 14.811            -1.0                                                       d.sub.4 variable                                                       r.sub.5 -25.147           0.846226                                       G.sub.3     d.sub.5 4.0                                                                         N.sub.3 1.4914                                                                       ν.sub.3 57.82                                          r.sub.6 -13.247                                                          Lo.sub.2    d.sub.6 1.0                                                            r.sub.7 692.674                                                          G.sub.4     d.sub.7 2.5                                                                         N.sub.4 1.4914                                                                       ν.sub.4 57.82                                          r.sub.8 -16.52                                                                       d.sub.8 variable                                                       r.sub.9 14.3              -0.6                                           Lc G.sub.5  d.sub.9 3.4                                                                         N.sub.5 1.4914                                                                       ν.sub.5 57.82                                          r.sub.10 -75.00                                                                      d.sub.10 25.5                                                          r.sub.11 -94.616                                                         G.sub.6     d.sub.11 1.3                                                                        N.sub.6 1.584                                                                        ν.sub.6 31.00                                          r.sub.12 9.209                                                           Le          d.sub.12 0.5                                                           r.sub.13 8.00             -0.15                                          G.sub.7     d.sub.13 3.7                                                                        N.sub.7 1.4914                                                                       ν.sub.7 57.82                                          r.sub.14 -13.181                                                                     d.sub.14 13.0                                                          r.sub.15 pupil                                                           __________________________________________________________________________                          diopter                                                                            finder                                                       d.sub.4                                                                           d.sub.8 in center                                                                          magnification                                      __________________________________________________________________________    Standard  23.05                                                                             27.52   -0.998                                                                             0.468                                              Middle    6.14                                                                              39.37   -0.997                                                                             0.936                                              Tele      0.5 51.22   -1.003                                                                             1.403                                              frame                 -1.024                                                  __________________________________________________________________________    1/f.sub.1  = -0.0370   1/f.sub.2                                                                         = 0.0499                                           f.sub.e /(f.sub.1 · f.sub.2)                                                    = -0.0555   r.sub.1B /r.sub.1F                                                                = 0.5072                                           r.sub.2B /r.sub.2F                                                                       = 0.6570    f.sub.oT /f.sub.e                                                                 = 1.5986                                           ν eP    = 57.82     ν eN                                                                           =  31.00                                           r.sub.eP /r.sub.eN                                                                       = 0.8687    r.sub.eB /f.sub.e                                                                 = -0.4390                                          f.sub.eN /f.sub.e                                                                        = -0.4764                                                          __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    Embodiment 5                                                                  __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 -148.203                                                         G.sub.1     d.sub.1 1.2                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 58.334            0.995124                                       Lo.sub.1    d.sub.2 2.0                                                            r.sub.3 -238.555                                                         G.sub.2     d.sub.3 1.2                                                                         N.sub.2 1.584                                                                        ν.sub.2 31.00                                          r.sub.4 23.77             0.964397                                                   d.sub.4 variable                                                       r.sub.5 -125.882                                                         G.sub.3     d.sub.5 4.0                                                                         N.sub.3 1.4914                                                                       ν.sub.3 57.82                                          r.sub.6 -14.978                                                          Lo.sub.2    d.sub.6 1.0                                                            r.sub.7 345.689                                                          G.sub.4     d.sub.7 2.5                                                                         N.sub.4 1.4914                                                                       ν.sub.4 57.82                                          r.sub.8 -26.247                                                                      d.sub.8 variable                                                       r.sub.9 14.3              -0.6                                           Lc G.sub.5  d.sub.9 3.4                                                                         N.sub.5 1.4914                                                                       ν.sub.5  57.82                                         r.sub.10 -75.00                                                                      d.sub.10 25.5                                                          r.sub.11 -94.616                                                         G.sub.6     d.sub.11 1.3                                                                        N.sub.6 1.584                                                                        ν.sub.6 31.00                                          r.sub.12 9.209                                                           Le          d.sub.12 0.5                                                           r.sub.13 8.00             -0.14                                          G.sub.7     d.sub.13 3.7                                                                        N.sub.7 1.4914                                                                       ν.sub.7 57.82                                          r.sub.14 -13.181                                                                     d.sub.14 13.0                                                          r.sub.15 pupil                                                           __________________________________________________________________________                           diopter                                                                            finder                                                      d.sub.4                                                                           d.sub.8  in center                                                                          magnification                                     __________________________________________________________________________    Standard  23.05                                                                             27.52    -0.998                                                                             0.468                                             Middle    6.14                                                                              39.37    -0.997                                                                             0.936                                             Tele      0.5 51.22    -1.003                                                                             1.403                                             frame                  -1.024                                                 __________________________________________________________________________    1/f.sub.1  = -0.0370   1/f.sub.2                                                                         = 0.0499                                           f.sub.e /(f.sub.1 · f.sub.2)                                                    = -0.0555   r.sub.1B /r.sub.1F                                                                = 0.5072                                           r.sub.2B /r.sub.2F                                                                       = 0.6570    f.sub.oT /f.sub.e                                                                 = 1.5986                                           ν eP    = 57.82     ν eN                                                                           = 31.00                                            r.sub.eP /r.sub. eN                                                                      = 0.8687    r.sub.eB /f.sub.e                                                                 = -0.4390                                          f.sub.eN /f.sub.e                                                                        = -0.4764                                                          __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Embodiment 6                                                                  __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 41.346                                                           G.sub.1     d.sub.1 1.2                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 27.047            0.750491                                       Lo.sub.1    d.sub.2 2.0                                                            r.sub.3 -79.748                                                          G.sub.2     d.sub.3 1.2                                                                         N.sub.2 1.584                                                                        ν.sub.2 31.00                                          r.sub.4 26.443            0.590166                                                   d.sub.4 variable                                                       r.sub.5 -212.904                                                         G.sub.3     d.sub.5 4.0                                                                         N.sub.3 1.4914                                                                       ν.sub.3 57.82                                          r.sub.6 -17.328           0.846806                                       Lo.sub.2    d.sub.6 1.0                                                            r.sub.7 67.021                                                           G.sub.4     d.sub.7 2.5                                                                         N.sub.4 1.4914                                                                       ν.sub.4 57.82                                          r.sub.8 -28.8             0.869546                                                   d.sub.8 variable                                                       r.sub.9 14.3              -0.6                                           Lc G.sub.5  d.sub.9  3.4                                                                        N.sub.5 1.4914                                                                       ν.sub.5 57.82                                          r.sub.10 -75.00                                                                      d.sub.10 25.5                                                          r.sub.11 -94.616                                                         G.sub.6     d.sub.11 1.3                                                                        N.sub.6 1.584                                                                        ν.sub.6 31.00                                          r.sub.12 9.209                                                           Le          d.sub.12 0.5                                                           r.sub.13 8.00             -0.15                                          G.sub.7     d.sub.13 3.7                                                                        N.sub.7 1.4914                                                                       ν.sub.7 57.82                                          r.sub.14 -13.181                                                                     d.sub.14 13.0                                                          r.sub.15 pupil                                                           __________________________________________________________________________                          diopter                                                                            finder                                                       d.sub.4                                                                           d.sub.8 in center                                                                          magnification                                      __________________________________________________________________________    Standard  23.4                                                                              26.49   -1.000                                                                             0.468                                              Middle     6.225                                                                            38.53   -1.000                                                                             0.936                                              Tele      0.5 50.57   -0.999                                                                             1.404                                              frame                 -1.024                                                  __________________________________________________________________________    1/f.sub.1  = -0.0370   1/f.sub.2                                                                         = 0.0492                                           f.sub.e /(f.sub.1 · f.sub.2)                                                    = -0.0546   r.sub.1B /r.sub.1F                                                                = 0.6395                                           r.sub.2B /r.sub.2F                                                                       = 0.1353    f.sub.oT /f.sub.e                                                                 = 1.5986                                           ν eP    =  57.82    ν eN                                                                           = 31.00                                            r.sub.eP /r.sub.eN                                                                       = 0.8687    r.sub.eB /f.sub.e                                                                 = -0.4390                                          f.sub.eN /f.sub.e                                                                        = -0.4764                                                          __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Embodiment 7                                                                  __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 29.038                                                           G.sub.1     d.sub.1 1.2                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 35.181            0.599136                                       Lo.sub.1    d.sub.2 2.0                                                            r.sub.3 -28.105                                                          G.sub.2     d.sub.3 1.2                                                                         N.sub.2 1.584                                                                        ν.sub.2 31.00                                          r.sub.4 28.253            0.740232                                                   d.sub.4 variable                                                       r.sub.5 -131.19                                                          G.sub.3     d.sub.5 4.0                                                                         N.sub.3 1.4914                                                                       ν.sub.3 57.82                                          r.sub.6 -17.96            0.675744                                       Lo.sub.2    d.sub.6 1.0                                                            r.sub.7 83.472                                                           G.sub.4     d.sub.7 2.5                                                                         N.sub.4 1.4914                                                                       ν.sub.4 57.82                                          r.sub.8 -22.843           0.466598                                                   d.sub.8 variable                                                       r.sub.9 14.3              -0.6                                           Lc G.sub.5  d.sub.9 3.4                                                                         N.sub. 5 1.4914                                                                      ν.sub.5 57.82                                          r.sub.10 -75.00                                                                      d.sub.10 25.5                                                          r.sub.11 -94.616                                                         G.sub.6     d.sub.11 1.3                                                                        N.sub.6 1.584                                                                        ν.sub.6 31.00                                          r.sub.12 9.209                                                           Le          d.sub.12 0.5                                                           r.sub.13 8.00             -0.0075                                        G.sub.7     d.sub.13 3.7                                                                        N.sub.7 1.4914                                                                       ν.sub.7 57.82                                          r.sub.14 -13.181                                                                     d.sub.14 13.0                                                          r.sub.15 pupil                                                           __________________________________________________________________________                          diopter                                                                            finder                                                       d.sub.4                                                                           d.sub.8 in center                                                                          magnification                                      __________________________________________________________________________    Standard  23.095                                                                            26.33   -1.000                                                                             0.468                                              Middle    6.15                                                                              38.21   -0.997                                                                             0.936                                              Tele      0.5 50.09   -0.997                                                                             1.404                                              frame                 -1.024                                                  __________________________________________________________________________    1/f.sub.1  = -0.0370   1/f.sub.2                                                                         = 0.0498                                           f.sub.e /(f.sub.1 · f.sub.2)                                                    = -0.0554   r.sub.1B /r.sub.1F                                                                = 0.9730                                           r.sub.2B /r.sub.2F                                                                       = 0.1741    f.sub.oT /f.sub.e                                                                 = 1.5986                                           ν eP    = 57.82     ν eN                                                                           = 31.00                                            r.sub.eP /r.sub.eN                                                                       = 0.8687    r.sub.eB /f.sub.e                                                                 = -0.4390                                          f.sub.eN /f.sub.e                                                                        = -0.4764                                                          __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Embodiment 8                                                                  __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 38.783                                                           G.sub.1     d.sub.1 1.2                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 24.348            5.283682                                       Lo.sub.1    d.sub.2 2.0                                                            r.sub.3 107.21                                                           G.sub.2     d.sub.3 1.2                                                                         N.sub.2 1.584                                                                        ν.sub.2 31.00                                          r.sub.4 17.939            1.749085                                                   d.sub.4 variable                                                       r.sub.5 -27.556                                                          G.sub.3     d.sub.5 4.0                                                                         N.sub.3 1.4914                                                                       ν.sub.3 57.82                                          r.sub.6 -24.709           0.712956                                       Lo.sub.2    d.sub.6 1.0                                                            r.sub.7 98.816                                                           G.sub.4     d.sub.7 2.5                                                                         N.sub.4 1.4914                                                                       ν.sub.4 57.82                                          r.sub.8 -13.343                                                                      d.sub.8 variable                                                       r.sub.9 14.3              -0.6                                           Lc G.sub.5  d.sub.9 3.4                                                                         N.sub.5 1.4914                                                                       ν.sub. 5 57.82                                         r.sub.10 -75.00                                                                      d.sub.10 25.5                                                          r.sub.11 -94.616                                                         G.sub.6     d.sub.11 1.3                                                                        N.sub.6 1.584                                                                        ν.sub.6 31.00                                          r.sub.12 9.209                                                           Le          d.sub.12 0.5                                                           r.sub.13 8.00             -0.3                                           G.sub.7     d.sub.13 3.7                                                                        N.sub.7 1.4914                                                                       ν.sub.7 57.82                                          r.sub.14 -13.181                                                                     d.sub.14 13.0                                                          r.sub.15 pupil                                                           __________________________________________________________________________                          diopter                                                                            finder                                                       d.sub.4                                                                           d.sub.8 in center                                                                          magnification                                      __________________________________________________________________________    Standard  25.556                                                                            30.47   -0.999                                                                             0.468                                              Middle    6.77                                                                              42.95   -0.991                                                                             0.936                                              Tele      0.5 55.42   -0.999                                                                             1.404                                              frame                 -1.024                                                  __________________________________________________________________________    1/f.sub.1  = -0.0360   1/f.sub.2                                                                         = 0.0462                                           f.sub.e /(f.sub.1 · f.sub.2)                                                    = -0.0499   r.sub.1B /r.sub.1F                                                                = 0.4625                                           r.sub.2B /r.sub.2F                                                                       = 0.4842    f.sub.oT /f.sub.e                                                                 = 1.5986                                           ν eP    = 57.82     ν eN                                                                           = 31.00                                            r.sub.eP /r.sub.eN                                                                       = 0.8687    r.sub.eB /f.sub.e                                                                 = -0.4390                                          f.sub.eN /f.sub.e                                                                        = -0.4764                                                          __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________    Embodiment 9                                                                  __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 30.00                                                            G.sub.1     d.sub.1 1.2                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 21.286            0.751266                                       Lo.sub.1    d.sub.2 2.0                                                            r.sub.3 -155.00                                                          G.sub.2     d.sub.3 1.2                                                                         N.sub.2 1.584                                                                        ν.sub.2 31.00                                          r.sub.4 23.28             0.634513                                                   d.sub.4 variable                                                       r.sub.5 60.00                                                            G.sub.3     d.sub.5 4.0                                                                         N.sub.3 1.62041                                                                      ν.sub.3 60.29                                     Lo.sub.2                                                                           r.sub.6 -10.00                                                           G.sub.4     d.sub.6 1.0                                                                         N.sub.4 1.6727                                                                       ν.sub.4 32.22                                          r.sub.7 -13.82                                                                       d.sub.7 variable                                                       r.sub.8 14.3              -0.6                                           Lc G.sub.5  d.sub.8 3.4                                                                         N.sub.5 1.4914                                                                       ν.sub.5 57.82                                          r.sub.9 -75.00                                                                       d.sub.9 25.5                                                           r.sub.10 -94.616                                                         G.sub.6     d.sub.10 1.3                                                                        N.sub.6 1.584                                                                        ν.sub.6 31.00                                          r.sub.11 9.209                                                           Le          d.sub.11 0.5                                                           r.sub.12 8.00             -0.15                                          G.sub.7     d.sub.12 3.7                                                                        N.sub.7 1.4914                                                                       ν.sub.7 57.82                                          r.sub.13 -13.181                                                                     d.sub.13 13.0                                                          r.sub.14 pupil                                                           __________________________________________________________________________                          diopter                                                                            finder                                                       d.sub.4                                                                           d.sub.7 in center                                                                          magnification                                      __________________________________________________________________________    Standard  22.421                                                                            25.0    -1.000                                                                             0.453                                              Middle    5.98                                                                              35.5    -0.996                                                                             0.891                                              Tele      0.5  45.66  -0.999                                                                             1.316                                              frame                 -1.024                                                  __________________________________________________________________________    1/f.sub.1  = -0.0368   1/f.sub.2                                                                         = 0.0525                                           f.sub.e /(f.sub.1 · f.sub.2)                                                    = -0.0580   r.sub.1B /r.sub.1F                                                                = 0.7760                                           r.sub.2B /r.sub.2F                                                                       = 0.2303    f.sub.oT /f.sub.e                                                                 = 1.4986                                           ν eP    = 57.82     ν eN                                                                           = 31.00                                            r.sub.eP /r.sub.eN                                                                       = 0.8687    r.sub.eB /f.sub.e                                                                 = -0.4390                                          f.sub.eN /f.sub.e                                                                        = -0.4764                                                          __________________________________________________________________________

                                      TABLE 10                                    __________________________________________________________________________    Embodiment 10                                                                 __________________________________________________________________________         Radius of                                                                            Axial Refractive                                                                           Abbe  Aspheric                                            Curvature                                                                            Distance                                                                            Index (Nd)                                                                           Number                                                                              Coefficient (ε)                        __________________________________________________________________________         r.sub.1 41.346                                                           G.sub.1     d.sub.1 1.2                                                                         N.sub.1 1.584                                                                        ν.sub.1 31.00                                          r.sub.2 27.047            0.750491                                       Lo.sub.1    d.sub.2 2.0                                                            r.sub.3 -79.748                                                          G.sub.2     d.sub.3 1.2                                                                         N.sub.2 1.584                                                                        ν.sub.2 31.00                                          r.sub.4 26.443            0.590166                                                   d.sub.4 variable                                                       r.sub.5 -212.904                                                         G.sub.3     d.sub.5 4.0                                                                         N.sub.3 1.4914                                                                       ν.sub.3 57.82                                          r.sub.6 -17.328                                                          Lo.sub.2    d.sub.6 1.0                                                            r.sub.7 67.021                                                           G.sub.4     d.sub.7 2.5                                                                         N.sub.4 1.4914                                                                       ν.sub.4 57.82                                          r.sub.8 -28.8                                                                        d.sub.8 variable                                                       r.sub.9 14.3              -1.3                                           Lc G.sub.5  d.sub.9 3.4                                                                         N.sub.5 1.4914                                                                       ν.sub.5 57.82                                          r.sub.10 -75.00                                                                      d.sub.10 20.5                                                          r.sub.11 -80.00                                                          G.sub.6     d.sub.11 1.1                                                                        N.sub.6 1.584                                                                        ν.sub.6 31.00                                          r.sub.12 13.452                                                          Le          d.sub.12 0.5                                                           r.sub.13 8.00             -0.71                                          G.sub.7     d.sub.13 4.0                                                                        N.sub.7 1.4914                                                                       ν.sub.7 57.82                                          r.sub.14 -16.00                                                                      d.sub.14 13.0                                                          r.sub.15 pupil                                                           __________________________________________________________________________                          diopter                                                                            finder                                                       d.sub.4                                                                           d.sub.8 in center                                                                          magnification                                      __________________________________________________________________________    Standard  23.4                                                                              26.49   -0.963                                                                             0.599                                              Middle     6.225                                                                            38.53   -0.961                                                                             1.198                                              Tele      0.5 50.57   -0.959                                                                             1.796                                              frame                 -1.000                                                  __________________________________________________________________________    1/f.sub.1  = -0.0370   1/f.sub.2                                                                         = 0.0492                                           f.sub.e /(f.sub.1 · f.sub.2)                                                    = -0.0423   r.sub.1B /r.sub.1F                                                                = 0.6395                                           r.sub.2B /r.sub.2F                                                                       = 0.1353    f.sub.oT /f.sub.e                                                                 = 2.0639                                           ν eP    = 57.82     ν eN                                                                           = 31.00                                            r.sub.eP /r.sub. eN                                                                      = 0.5947    r.sub.eB /f.sub.e                                                                 = -0.6880                                          f.sub.eN /f.sub.e                                                                        = -0.8442                                                          __________________________________________________________________________

                  TABLE 11                                                        ______________________________________                                        Wide            Middle       Tele                                             axial  diop.   finder   diop. finder diop. finder                             distance                                                                             in ctr. mag.     in ctr.                                                                             mag.   in ctr.                                                                             mag.                               ______________________________________                                        0.1    -4.301  0.501    -4.301                                                                              1.003  -4.301                                                                              1.504                              0.2    -3.461  0.503    -3.461                                                                              1.006  -3.461                                                                              1.509                              0.3    -2.616  0.505    -2.616                                                                              1.009  -2.616                                                                              1.514                              0.4    -1.765  0.506    -1.765                                                                              1.013  -1.765                                                                              1.519                              0.5    -0.909  0.508    -0.909                                                                              1.016  -0.909                                                                              1.524                              0.6    -0.047  0.510    -0.047                                                                              1.019  -0.047                                                                              1.529                              0.7     0.821  0.511     0.821                                                                              1.023   0.821                                                                              1.534                              0.8     1.695  0.513     1.695                                                                              1.026   1.695                                                                              1.540                              0.9     2.575  0.515     2.575                                                                              1.030   2.575                                                                              1.545                              ______________________________________                                    

What is claimed is:
 1. Variable magnification viewfinder optical system,comprising from the object side:an objective lens group of a positiverefractive power, including a first lens unit of a negative refractivepower and a second lens unit of a positive refractive power, said firstand second lens units being shiftable along the optical axis of theoptical system for changing the magnification of the optical system; acondenser lens group of a positive refractive power; and an eyepiecelens group of a positive refractive power; wherein the objective lensgroup forms a real image of an object and the real image can be observedthrough the eyepiece lens group; and wherein the optical system fulfillsthe following conditions;

    -0.09/mm<1/f.sub.1 <-0.03/mm

    0.04/mm<1/f.sub.2 <0.11/mm

    -0.1/mm<f.sub.e /(f.sub.1 ·f.sub.2)<-0.035/mm

wherein:f₁ represents the focal length of the first lens unit of theobjective lens group in millimeters; f₂ represents the focal length ofthe second lens unit of the objective lens group in millimeters; andf_(e) represents the focal length of the eyepiece lens group inmillimeters.
 2. Variable magnification viewfinder optical system asclaimed in claim 1, wherein the optical system further fulfills thefollowing conditions;

    |r.sub.1B /r.sub.1F |<6

    |r.sub.2B /r.sub.2F |<2

    0.9<f.sub.oT /f.sub.e <3

wherein: r_(1B) represents a radius of curvature of a lens surfacelocated at the object side in the first lens unit of the objective lensgroup; r_(1F) represents a radius of curvature of a lens surface locatedat the eye side in the first lens unit of the objective lens group;r_(2B) represents a radius of curvature of a lens surface located at theobject side in the second lens unit of the objective lens group; r_(2F)represents a radius of curvature of a lens surface located at the eyeside in the second lens unit of the objective lens group; and f_(oT)represents the focal length of the objective lens group at the largestmagnification condition.
 3. Variable magnification viewfinder opticalsystem as claimed in claim 1, wherein the eyepiece lens group consistsof one positive lens element and one negative lens element, and whereinthe optical system further fulfills the following conditions;

    ν.sub.eP >45

    ν.sub.eN <40

    0.5<r.sub.eP /r.sub.eN <0.95

    -0.8<r.sub.eB /f.sub.e <-0.35

    -0.9<f.sub.eN /f.sub.e <-0.4

wherein: ν_(eP) represents Abbe number of the positive lens elementincluded in the eyepiece lens group; ν_(eN) represents Abbe number ofthe negative lens element included in the eyepiece lens group; r_(eP)represents a radius of curvature of a lens surface of the positive lenselement faced to the negative lens element; r_(eN) represents a radiusof curvature of a lens surface of the negative lens element faced to thepositive lens element; r_(eB) represents a radius of curvature of a lenssurface located at the eye side in the eyepiece lens group; and f_(eN)represents the focal length of the negative lens element of the eyepiecelens group.
 4. Variable magnification viewfinder optical system asclaimed in claim 3, wherein one of the positive and negative lenselements in the eyepiece lens group is shiftable for adjusting thediopter of the optical system.
 5. Variable magnification viewfinderoptical system as claimed in claim 1, wherein the objective lens groupincludes at least one aspheric surface.
 6. Variable magnificationviewfinder optical system as claimed in claim 1, wherein the condenserlens group includes at least one aspheric surface whose radius ofcurvature decreases in proportional to the height measured from theoptical axis in a direction perpendicular to the optical axis. 7.Variable magnification viewfinder optical system as claimed in claim 1,wherein the eyepiece lens group includes at least one aspheric surfacewhose radius of curvature decreases in proportional to the heightmeasured from the optical axis in a direction perpendicular to theoptical axis.
 8. Variable magnification viewfinder optical system,comprising from the object side:objective lens group of a positiverefractive power, including a first lens unit of a negative refractivepower and a second lens unit of a positive refractive power, said firstand second lens units being shiftable along the optical axis of theoptical system for changing the magnification of the optical system;condenser lens group of a positive refractive power; and eyepiece lensgroup of a positive refractive power; wherein the objective lens groupforms a real image of an object and the real image can be observedthrough the eyepiece lens group; and wherein the optical system fulfillsthe following condition;

    0.9<f.sub.oT /f.sub.e <3

wherein: f_(oT) represents a focal length of the objective lens group atthe largest magnification condition; and f_(e) represents a focal lengthof the eyepiece lens group.
 9. Variable magnification viewfinder opticalsystem, comprising from the object side:objective lens group of apositive refractive power, including a first lens unit of a negativerefractive power and a second lens unit of a positive refractive power,said first and second lens units being shiftable along the optical axisof the optical system for changing the magnification of the opticalsystem; condenser lens group of a positive refractive power; andeyepiece lens group of a positive refractive power, consisting of onepositive lens element and one negative lens element; wherein theobjective lens group forms a real image of an object and the real imagecan be observed through the eyepiece lens group.
 10. Variablemagnification viewfinder optical system as claimed in claim 9, whereinthe optical system further fulfills the following conditions;

    ν.sub.eP >45

ν_(eN) <40 wherein; ν_(eP) represents Abbe number of the positive lenselement included in the eyepiece lens group; and ν_(eN) represents Abbenumber of the neative lens element included in the eyepiece lens group.11. Variable magnification viewfinder optical system as claimed in claim10, wherein the eyepiece lens group consists of, from the object side,one positive lens element and one negative lens element.
 12. Variablemagnification viewfinder optical system as claimed in claim 11, whereinthe optical system further fulfills the following conditions;

    0.5<r.sub.eP /r.sub.eN <0.95

    -0.8<r.sub.eB /f.sub.e <-0.35

    -0.9<f.sub.eN /f.sub.e <-0.4

wherein: r_(eP) represents a radius of curvature of a lens surface ofthe positive lens element faced to the negative lens element; r_(eN)represents a radius of curvature of a lens surface of the negative lenselement faced to the positive lens element; r_(eB) represents a radiusof curvature of a lens surface located at the eye side in the eyepiecelens group; f_(eN) represents a focal length of the negative lenselement of the eyepiece lens group; and f_(e) represents a focal lengthof the eyepiece lens group.
 13. Variable magnification viewfinderoptical system as claimed in claim 9, wherein one of the positive andnegative lens elements in the eyepiece lens group is shiftable foradjusting the diopter of the optical system.
 14. Variable magnificationviewfinder optical system as claimed in claim 13, wherein the positivelens element of the eyepiece lens group is shiftable for adjusting thediopter of the optical system.
 15. Variable magnification viewfinderoptical system as claimed in claim 13, wherein the eyepiece lens groupincludes at least one aspheric surface whose radius of curvaturedecreases in proportional to the height measured from the optical axisin a direction perpendicular to the optical axis.
 16. Variablemagnification viewfinder optical system, comprising from the objectside:objective lens group of a positive refractive power, including afirst lens unit of a negative refractive power and a second lens unit ofa positive refractive power, said first and second lens units beingshiftable along the optical axis of the optical system for changing themagnification of the optical system; image reverse optical systemincluding a condenser lens group of a positive refractive power; andeyepiece lens group of a positive refractive power, consisting of onepositive lens element and one negative lens element; wherein theobjective lens group forms a real image of an object and the real imagecan be observed through the eyepiece lens group.
 17. Variablemagnification viewfinder optical system as claimed in claim 16, whereinthe objective lens group forms the real image out of the condenser lensgroup.
 18. Variable magnification viewfinder optical system as claimedin claim 16, wherein the image reverse optical system is a parro prism.